1. Field of the Invention
The present invention relates to a steering wheel used for a steering apparatus of a vehicle.
2. Description of the Related Art
Generally, in the core bar of a steering wheel for a vehicle, a ring shaped hub, a rim mounted to a steering shaft, and a spoke connecting the hub and the rim to each other are integrally molded with each other by casting using metal materials such as alloy including magnesium or aluminum and the like.
Further, in a steering wheel (especially in a steering wheel having three spokes), rigidity (strength) at portions of the spokes corresponding to the abdomen of a vehicle occupant seated on a driver's seat is set and the spokes are deformed by a load at a predetermined value or more from the vehicle occupant side. Accordingly, for example, in a case in which the body of the vehicle occupant has moved and pressed the steering wheel due to inertia at the time of a rapid deceleration of a vehicle, the spokes are deformed in association with the rim so as to absorb kinetic energy from the moving body of the vehicle occupant.
Moreover, the setting of strength of spokes described above is effected even in a case in which an air bag apparatus is installed at a steering wheel. Namely, even in a case that the body (i.e., the abdomen) of the vehicle occupant has pressed the steering wheel by a pressing force at a predetermined value or more via the inflated bag body, the spokes are deformed in association with the rim so that kinetic energy from the vehicle occupant is absorbed by the spokes and the inflated bag body.
However, in a case that a steering wheel is integrally molded by casting, because types of dies drawn from a cast mold and the like may restrict the configuration and/or dimension of a spoke and the rigidity of the spoke must be set/determined by considering this restriction, there has been a problem in that freedom in designing a steering wheel is extremely low. Further, during coagulation in casting, because casts are not cooled uniformly, shrinkage during the coagulation may partly be controlled. For this reason, it is difficult to set the strength of a spoke, and at the development stage of the product, a fine tuning must be effected for the configuration and/or thickness of the spoke in order to set the spoke at a predetermined rigidity. In addition, since a cast model for the product or a cast mold must be adjusted for each tuning, there has been a problem in that the manufacturing cost from a development to a quantity production becomes expensive.